What can we do if the courier company failed to deliver all the goods we ordered?

Our Issue is with the courier company. We have an account with that courier company.

We had recently booked the courier to pick goods from France and deliver them to our shop in Dubai.

So it was total of 50 carton (200pcs) which they had to pick and deliver to us. They picked 50 cartons but delivered only 47 cartons.

And now they don't know where those cartons are and because of that, we are facing issues with our customer.

When we call the courier they are not replying and when we email them they are telling us that they are investigating. But still there has been no result of the investigation.

So please advise what is the procedure to move with this.